In a large mixing bowl, combine the rolled oats, protein powder, and chia seeds or flaxseeds (if using).
In a microwave-safe bowl, gently heat the peanut butter and honey (or maple syrup) for 20-30 seconds until they are easy to stir together.
Pour the peanut butter and honey mixture into the dry ingredients. Add the milk and stir until everything is well combined. The mixture should be sticky and slightly firm.
Line a baking dish (8x8-inch) with parchment paper. Pour the mixture into the dish and press it down evenly with the back of a spoon.
If you're using chocolate chips, melt them in the microwave (in 15-20 second bursts) and drizzle or spread them over the top of the bars.
Place the dish in the fridge for at least 1-2 hours to let the bars firm up.
Once firm, lift the mixture out of the dish using the parchment paper and cut it into bars. Store in the fridge for up to a week